This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Endogenous MRI perfusion methods such as ASL require accurate calibration of the transmitted radiofrequency magnetic field (B1) in order to achieve precise inversion of the labeled spins. This is especially necessary at high main magnetic field (B0) strengths, where wavelength and penetration effects create heterogeneous B1 fields as well as high local SAR effects. B1 heterogeneity at high field is further compounded by increased B0 inhomogeneity, which can result in underestimation of the transmit B1 field by standard methods, including AFI. When the degree of off-resonance is appreciable compared to the B1 amplitude, then the nominal flip angle measured by the AFI method is less than the actual nutation produced by the B1 field. In order to obtain the correct B1 field strength, the B0 field must also be measured, and the complete system described by the Bloch equations can be solved for the actual flip angle, which is the true measure of the B1 field.